The present invention relates to the manufacture of glass articles having predetermined composition profiles, and more particularly to the manufacture of doped silica tubes having predetermined radial composition profiles of B.sub.2 O.sub.3 and fluorine.
Glass tubes having predetermined radial composition profiles are utilized in the manufacture of various optical devices. For example, doped silica tubes having refractive indices lower than that of silica are used in the manufacture of a type of fiber optic coupler referred to as a multiclad coupler. Such couplers are formed by inserting into a glass tube at least a portion of each of a plurality of optical fibers so that the fiber portions occupy the midregion of the tube. The tube midregion is collapsed onto fibers, and the central portion of the midregion is stretched until a predetermined coupling occurs between the fibers. Couplers having various kinds of coupling characteristics, e.g. WDM, achromatic, and the like, have been made by this process. See, for example, U.S. Pat. Nos. 4,931,076 and 5,011,251.
The physical characteristics of the glass tube affect both the manufacturing process and the optical characteristics of the resultant coupler. Such physical characteristics include viscosity, refractive index and thermal coefficient of expansion (TCE).
In multiclad fiber optic couplers, the tube refractive index n.sub.3 must be lower than the refractive index n.sub.2 of the fiber cladding. Commercially available single-mode optical fibers usually have a value of n.sub.2 that is equal to or near that of silica. If silica is employed as the base glass for the tube, a dopant is added thereto for the purpose of decreasing n.sub.3 to a value lower than n.sub.2.The refractive index of the tube relative to that of the cladding is represented by .DELTA..sub.2-3, the value of which is defined as EQU .DELTA..sub.2-3 =(n.sub.2.sup.2 -n.sub.3.sup.2)/2n.sub.2.sup.2
The value of .DELTA..sub.2-3 for standard WDM couplers has usually been between 0.26% and 0.35%. During the manufacture of achromatic overclad couplers, process reproducibility is enhanced by employing preforms having .DELTA..sub.2-3 values above 0.35%. When employed in the manufacture of fiber optic couplers, the refractive index of that half of the tube having the smaller radius is more critical since it is adjacent to the optical fibers and therefore propagates the coupled signal in the coupling region.
The dopants B.sub.2 O.sub.3 and fluorine have been employed to lower the refractive index of silica tubes, fluorine having the greater effect on refractive index. They also advantageously decrease the tube viscosity to a value lower than that of the coupler fibers, boron having the greater effect on viscosity. This enhances to a certain extent the collapsing of the tube onto the fibers; the tube glass flows around the fibers without distorting their shape. These dopants also affect the TCE of the tube, boron increasing the TCE relative to silica, while fluorine reduces the TCE relative to silica when used in concentrations needed for couplers. The TCE of the tube can affect the polarization sensitivity of the resultant coupler. The TCE of the tube is usually tailored to be compatible with that of the fiber cladding. For a specific coupler, the concentration of boron and fluorine in the glass tube are selected to provide the optimal combination of the above-discussed optical and mechanical characteristics.
Glass coupler tubes have been made by a flame oxidation process involving the following steps: (1) deposition of glass particles on a mandrel, (2) mandrel removal, and (3) consolidation of the resultant porous preform. That process has been used to make coupler tubes comprising SiO.sub.2 doped with 1 to 12 wt. % B.sub.2 O.sub.3, SiO.sub.2 doped with 0.1 to approximately 2.5 wt. % fluorine, and SiO.sub.2 doped with combinations of B.sub.2 O.sub.3 and fluorine. To achieve the desired combination of physical properties needed for a particular coupler, a combination of both dopants is usually needed. The boron has conventionally been added to the glass particle stream during the particle deposition step. However, when the required amount of fluorine exceeds about 0.6 wt. % (0.2% .DELTA.), the fluorine cannot be incorporated into the preform during the particle deposition step since fluorine in the reactant vapor stream reacts with water in the air to form HF. Also, it has been found that the addition of a fluorine-containing compound to the reactant stream emitted by a flame hydrolysis burner tends to decrease the rate of deposition of glass particles collected on the mandrel. Fluorine has therefore been added to the glass during the consolidation step.
The standard process for producing coupler tubes containing both boron and fluorine is as follows. A porous preform having a uniform radial composition of B.sub.2 O.sub.3 -doped SiO.sub.2 is formed on a mandrel by a process such as that disclosed in U.S. Pat. No. 4,165,223 and patents cited therein. The reactants, SiCl.sub.4 and BCl.sub.3 are supplied to a flame hydrolysis burner that directs a stream of B.sub.2 O.sub.3 -doped SiO.sub.2 particles toward the mandrel. After a coating of sufficient thickness is built up, the mandrel is removed, and the resultant tubular porous preform is placed in a consolidation furnace. The preform is then doped with fluorine by flowing into the preform aperture a mixture of He and a source of fluorine such as SiF.sub.4, CF.sub.4 or the like. These fluorine-containing reactants are less corrosive than fluorine gas, and they effectively dissociate to active fluorine in the consolidation furnace. The radial composition profiles of the resultant tubes are typically as illustrated in FIG. 1. The low concentration of B.sub.2 O.sub.3 near the tube aperture is the result of the reaction of the B.sub.2 O.sub.3 in the porous preform with F in the gas mixture to form BF.sub.3 which volatilizes and is exhausted from the furnace. This composition profile demonstrates the poor stability of the tube making process. Reproducibility of the process of manufacturing overclad couplers depends upon the uniformity of the radial composition profile of the overclad tube.
Attempts have been made to dope a pure silica porous preform with both boron and fluorine by flowing BF.sub.3 into the preform aperture while the preform is heated in a consolidation furnace. The resultant radial composition profile is shown in FIG. 2. While the radial concentration of fluorine is relatively uniform, the radial concentration of B.sub.2 O.sub.3 is unacceptable since it is much higher at small tube radii than at large tube radii.